For high temperature processing using a wafer boat in a vertical furnace, a pedestal is commonly used at a lower end of the furnace to support the wafer boat and to provide a thermal insulation plug at the bottom of the furnace. Such a pedestal typically includes an insulating material inside a quartz envelope, with the quartz envelope supporting the wafer boat and the insulating material providing thermal insulation.
A furnace with such a pedestal is illustrated in FIG. 1A. The furnace in its entirety is indicated by reference numeral 1 and is provided with a pedestal 90. A process tube 10 includes a gas infeed tube 12 to feed gas into tube 10 at a top end thereof and the tube 10 is supported by flanges 14. Gas is exhausted from a lower end of tube 10 by a gas exhaust tube, not shown. The cylindrical process tube 10 is surrounded by a cylindrical heating coil 20, an insulation material 30 and an outer shell 40. A metal doorplate 50 supports a stationary quartz door plate 60, and includes a boat rotation bearing 70 in its center region. The bearing 70 supports a rotating quartz door plate 80, which supports the pedestal 90. The pedestal 90 supports the wafer boat 200. The pedestal 90 and the wafer boat 200 can be inserted and removed from the furnace 1 with the aid of an elevation mechanism (not shown). The wafer boat 200 protrudes into a batch process chamber or reaction space 16, i.e., the volume inside the furnace 1 in which process gases can interact with wafers (not shown) during semiconductor fabrication processes. It will be appreciated that in the illustrated furnace 1, the process tube 10, the pedestal 90 and the door plates together delimit the reaction space 16.
The insulating material inside the pedestal 90 is typically a material with a high fraction of pores, such as ceramic foam or ceramic fibrous material. The material can be various materials, such as quartz foam, Al2O3 with high porosity or a mixture of high porosity materials. Because of the high fraction of open pores and the large surface area of the insulating material, exposing the material to process gases is undesirable and the insulating material is typically placed in a quartz envelope 92. However, upon loading the pedestal 90 into a hot furnace such as the furnace 1, the pedestal 90 heats up and gases present in the quartz envelope 92 expand. With the process chamber at a temperature of, e.g., about 1000° C., and with the internal gas pressure inside the pedestal 90 at about atmospheric pressure at room temperature, the pedestal will likely explode due to an increase in the internal gas pressure caused by heating.
To prevent this explosion, one possible solution is to evacuate the pedestal during manufacturing. However, this is complicated and the pedestal might not remain gas tight during use, with possible disastrous consequences such as explosion if gas leaks into the pedestal and later expands.
Another possible solution is to provide a pressure relief opening in the quartz envelope, so that, upon expansion of the gas inside the pedestal, the gas can escape from the pedestal into the process chamber, so that the pressure inside the pedestal remains at about atmospheric pressure. The pressure relief opening can be provided at the lower side of the pedestal, close to the gas exhaust and relatively far from the wafers. Unfortunately, it appears that together with the expanding gas, many particles also escape from the interior of the pedestal into the process chamber. The particles are difficult to confine to the bottom of the pedestal and can eventually distribute throughout the process chamber. These particles can be detected on wafers processed in the process chamber, which is undesirable since the particles can degrade process results. Undesirably, the insulating material can serve as an infinite source of particles for contaminating wafers.
Accordingly, there is a need for furnace pedestals that are resistant to exploding and that are not sources of contaminating particles.
According to one aspect of the invention, the quartz envelope of the pedestal is provided with a pressure relief opening comprising a filter which is permeable for gas, allowing expanding gas to escape from the pedestal's interior, but which is not permeable for particles.
According to another aspect of the invention, a system for semiconductor processing is provided. The system comprises a batch process chamber and a wafer boat configured to be accommodated in the process chamber. A pedestal is provided for supporting the wafer boat in the process chamber. The pedestal comprises an envelope with an internal gas volume. A wall of the envelope comprises an exterior opening in gas communication with the internal volume. The exterior opening defines a flow path for gas exiting the internal volume. The pedestal further comprises a particulate filter disposed in the flow path.
According to yet another aspect of the invention, a pedestal is provided for supporting an overlying wafer boat in a process chamber of a semiconductor processing furnace. The pedestal comprises an envelope defining walls of the pedestal and an exterior opening on a surface of the envelope. A particulate filter is configured to filter gas flow through the opening.
According to another aspect of the invention, a method for semiconductor processing is provided. The method comprises providing a wafer boat containing a plurality of semiconductor substrates. The wafer boat is supported on a hollow pedestal. Particles are filtered out from gas exiting an interior of the pedestal.